Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: paul.fr on 11/05/2007 21:58:38

Title: table cloth magic trick.
Post by: paul.fr on 11/05/2007 21:58:38

I'm sure you have all seen a magician, or your dad do the trick where they give a sharp tug on a table cloth and all the cups stay on the table. But the cloth is removed.

What's the physics behind that?

Title: table cloth magic trick.
Post by: another_someone on 11/05/2007 22:19:23

There are two different issues - stiction and inertia.

Stiction is a form of friction:

http://en.wikipedia.org/wiki/Stiction

Quote

Stiction is an informal portmaneau of the term "static friction" ("μs"), perhaps also influenced by the verb "stick".

Two solid objects pressing against each other (but not sliding) will require some threshold of force parallel to the surface of contact in order to overcome static cohesion. Stiction is a threshold, not a continuous force.

The stiction holds the cups to the table cloth, while the inertia is preventing the cups from moving.

If you pull the table cloth gently, you need relatively little force to pull the cups with you, and that force is insufficient to overcome the stiction between the cups and the tablecloth.

If you give a sharp tung on the tablecloth, the force required to overcome the inertia of the cups is significantly more, and that force exceeds the stiction between the cups and the tablecloth, so rather than the cup moving with the table cloth, the cups lose there adhesion to the tablecloth and do not move as the table cloth is pulled from under them.

Title: table cloth magic trick.
Post by: daveshorts on 13/05/2007 00:46:09

The other thing that helps about stiction is that it is a lot larger than moving friction so once you have the table cloth moving under the cups, the force on them drops. This is why skids are a bad thing in a car, there is a lot more grip if your wheels are rolling with the road than if they are not.

Title: table cloth magic trick.
Post by: lyner on 13/05/2007 12:06:43

Quote

the inertia is preventing the cups from moving.

'Inertia' is a dodgy term to use because its meaning is somewhere between Mass and Momentum, both of which are fully defined.
The term ' inertia' is not really needed, or even used, in simple  (Newtonian) mechanics.
It is the MASS of the object that is responsible for the trick.
The sideways force from friction with the table cloth acts for a short time. This 'impulse' (which is the force times the time) will give the object some momentum (mass times velocity). If the time is short, the final velocity will be small and it will soon slow down once  the tablecloth has gone and the friction with the table starts to act.
If you pull slowly, the force acts for longer and the object's speed will end up embarrassingly high. Crash.